Honestly, the only thing I can think is, if my skills are so OP that you have to make them never work, why let me get them?
Never work against
bosses. Dackly didn't bother fighting the horde of mooks who WEREN'T immune to being petrified or Demi'd, last I saw.
The thing is, Sachi isn't even our characters, we're much less likely to remember to use her, and OOCly, we should really know this stuff in advance so that we don't build characters that turn out completely useless in boss fights (see D&G). That's separate from weaknesses/immunities that SHOULD be learned ICly.
Stuff that doesn't work because of the [BOSS] tag (meta) is a different thing from stuff that doesn't work because of the creature type (IC), is I guess my position here.
-Really isn't my problem if you don't remember to use Sachi, then. I'm sorry to be harsh, but if you're forgetting to use the convenient tool NPC, then that's not really my problem anymore.
-No, you shouldn't be learning the boss weaknesses. They're not meant to be easy curbstomps, they're meant to be actual fights. Their immunities are in-universe immunities, not because of meta-considerations, though those may align of course.